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Modifying the Surface Properties ofSuperparamagnetic Iron OxideNanoparticles through A SolGel

ApproachYu Lu, Yadong Yin, Brian T. Mayers, and Younan Xia*

Department of Materials Science and Engineering, Department of Chemistry,

UniVersity of Washington, Seattle, Washington 98195

Received November 28, 2001; Revised Manuscript Received December 20, 2001

ABSTRACT

This paper describes a solgel approach for the coating of superparamagnetic iron oxide nanoparticles with uniform shells of amorphous

silica. The coating process has been successfully applied to particles contained in a commercial ferrofluid (e.g., the EMG 304 of Ferrofluidics)and those synthesized through a wet chemical process. The thickness of silica coating could be conveniently controlled in the range of 2100

nm by changing the concentration of the solgel solution. Fluorescent dyes, for example, 7-(dimethylamino)-4-methylcoumarin-3-isothiocyanate

(DACITC) and tetramethylrhodamine-5-isothiocyanate (5-TRITC), have also been incorporated into the silica shells by covalently coupling

these organic compounds with the solgel precursor. These multifunctional nanoparticles are potentially useful in a number of areas because

they can be simultaneously manipulated with an externally applied magnetic field and characterized in situ using conventional fluorescence

microscopy.

This paper describes a sol-gel method based on the

hydrolysis of tetraethyl orthosilicate (TEOS) for coating iron

oxide nanoparticles with conformal, uniform shells. The

thickness of these silica shells could be tuned from 2 up

to100 nm simply by varying the concentration of the sol-gel precursor. Fluorescent dyes could also be incorporated

into these silica shells through a covalent coupling between

these organic dyes and the sol-gel precursor.

Magnetic nanoparticles of iron oxides have been exten-

sively exploited as the materials of choice for ferrofluids, 1

high-density information storage,2 magnetic resonance imag-

ing (MRI),3 tissue-specific releasing of therapeutic agents,4

labeling and sorting of cells,5 and separation of biochemical

products.6 Most of these applications require the nanopar-

ticles to be chemically stable, uniform in size, and well-

dispersed in liquid media. As a result of anisotropic dipolar

attraction, pristine nanoparticles of iron oxides tend to

aggregate into large clusters and thus lose the specificproperties associated with single-domain, magnetic nano-

structures. Surfactants with relatively high concentrations are

often required to prevent such a aggregation. The presence

of large amounts of surfactants in these systems may severely

interfere with the medical and biological applications. In

addition, the reactivity of iron oxide particles has been shown

to greatly increase as their dimensions are reduced, and

particles relatively small in size may undergo rapid biodeg-

radation when they are directly exposed to biological

environments. It has been demonstrated that the formation

of a passive coating of inert materials such as silica on the

surfaces of iron oxide nanoparticles could help prevent theiraggregation in liquid and improve their chemical stability.7

Another advantage for the silica coating is that this surface

is often terminated by a silanol group that can react with

various coupling agents to covalently attach specific ligands

to the surfaces of these magnetic nanoparticles.8 Such a

capability will open the door to the design and synthesis of

magnetic carriers that can be used to deliver specific ligands

to target organs via the antibody-antigen recognition.

Two different approaches have been explored to generate

silica coatings on the surfaces of iron oxide particles. The

first method relied on the well-known Stober process,9 in

which silica was formed in situ through the hydrolysis and

condensation of a sol-gel precursor. This method was

originally applied to ferromagnetic rod-like nanoparticles,10

then to micrometer-sized hematite colloids by Matijevic and

co-workers,11 and later extended to other iron oxide nano-

particles by a number of research groups.12 Recently, this

method was further explored by several groups to form silica

shells on nanoparticles of metals such as gold and silver. 13

The other method was based on microemulsion synthesis,14

in which micelles or inverse micelles were used to confine

and control the coating of silica on core nanoparticles. This* To whom correspondence should be addressed: xia@

chem.washington.edu

NANO

LETTERS

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method might require much effort to separate the core-shell

nanoparticles from the large amount of surfactants associated

with the microemulsion system.Our initial effort was focused on the coating of super-

paramagnetic nanoparticles contained in commercial ferro-

fluids, for example, EMG 304 of Ferrofluids (Nashua, NH),

a water-based dispersion of iron oxide particles with dimen-

sions in the range of 5-15 nm. These particles were

stabilized by adding surfactants (such as oleic acid) to the

dispersion medium. Figure 1A shows the TEM image of

some particles that were extracted from EMG 304 by solvent

evaporation. High-resolution TEM (HRTEM) studies indicate

that there are probably two types of iron oxide particles in

this dispersion: maghemite (-Fe2O3, Figure 1B) and

magnetite (Fe3O4, Figure 1C).15 The coexistence of maghemite

and magnetite could be attributed to the oxidation of Fe3O4to -Fe2O3 during the synthesis.16 Both nanoparticles are

single crystalline in structure, and each of them is made of

one single magnetic domain. As a result, they exhibit the

superparamagnetic behavior and only possess a magnetic

moment in the presence of an external magnetic field. 17 When

the magnetic field is removed, these nanoparticles will return

to their nonmagnetic states immediately.

These magnetic nanoparticles could be directly coated with

amorphous silica, produced via the hydrolysis of a sol-gel

precursor. Because the iron oxide surface has a strong affinity

toward silica, no primer was required to promote the

deposition and adhesion of silica. In a typical procedure, 0.3

mL water-based ferrofluid (EMG 340) was diluted with 4

mL deionized (DI) water and 20 mL 2-propanol. Under

continuous mechanical stirring, 0.5 mL ammonia solution

(30wt %, Aldrich) and various amounts of TEOS (Aldrich,

used as-received) were consecutively added to the reaction

mixture. The reaction was allowed to proceed at room

temperature for 3 h under continuous stirring. The growth

of silica shells on iron oxide nanoparticles involved the base-

catalyzed hydrolysis of TEOS and subsequent condensation

of silica onto the surfaces of iron oxide cores. The core-shell nanoparticles could be separated from the reaction

medium by centrifuging at 4000 rpm and then redispersed

into DI water. Due to the presence of negative charges on

the surfaces of silica shells, these magnetic nanoparticles

having a core-shell structure could form very stable disper-

sions in water without adding other surfactants. The ratio

between the concentrations of iron oxide nanoparticles and

TEOS had been optimized to avoid the homogeneous

nucleation of silica and thus the formation of core-free silica

spheres.

Although several parameters (such as the growth time and

the concentration of ammonia catalyst or water10) could be

employed to control the thickness of silica shell, we found

it most convenient and reproducible to adjust the shell

thickness by changing the concentration of TEOS precursor.

Figure 2A-C shows the TEM images of iron oxide nano-

particles whose surfaces had been coated with silica shells

using different TEOS concentrations. Note that the silica shell

was homogeneous on each individual iron oxide particle,

regardless of its original morphology. As a result, the shape

of each iron oxide nanoparticle was essentially retained

during silica coating, especially when the shell was relatively

thin (Figure 2A). In this case, the polydispersity of the

Figure 1. (A) A TEM image of the superparamagnetic iron oxidenanoparticles contained in the ferrofluid EMG 304. (B) A HRTEMimage of the nanoparticle whose infringe spacings match those ofmaghemite (-Fe2O3). The lattice spacings for the (113) and (220)planes are 0.48 and 0.29 nm, respectively. (C) A HRTEM imageof the nanoparticle that could be assigned as magnetite (Fe3O4).The indicated (220) planes are separated from each other by 0.29nm.

Figure 2. (A-C) TEM images of iron oxide nanoparticles whosesurfaces have been coated with silica shells of various thicknesses.In this case, the thickness of silica coating could be adjusted bycontrolling the amount of precursor added to the solution: (A) 10,(B) 60, and (C) 1000 mg of TEOS to 20 mL of 2-propanol. (D) AHRTEM image of the iron oxide nanoparticle whose surface has

been uniformly coated with 6 nm of amorphous silica shell.

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original nanoparticles was also maintained. When the thick-

ness of silica coating was increased, the core-shell nano-

particles became more monodispersed because of a reduction

in the relative size distribution. Figure 2D shows the HRTEM

image of a silica-coated iron oxide nanoparticle. This image

clearly indicates the single crystallinity of the iron oxide core

and the amorphous nature of the silica shell. An examination

on the interface between iron oxide and silica suggests the

formation of a conformal coating of silica on the nanoparticle

core. Due to the presence of a homogeneous structure onthe core surface and a strong chemical affinity between iron

oxide and silicate, it was possible to generate such a core-

shell nanoparticle (of any specific size) with the iron oxide

nanoparticle encapsulated in the center. Aggregation between

iron oxide nanoparticles prior to or during the coating process

sometimes led to the trapping of multiple nuclei in a single

silica shell. Typical distributions for nuclei per shell were

on the order of 70% monomers, 19% dimers, 7% trimers,

and 5% greater than trimers. An increased ratio of monomers

may be favored through decreased iron oxide concentration

and good mixing by sonication to ensure that the core

particles are well separated before coating begins.Fluorescent iron oxide-silica nanoparticles have also been

synthesized by incorporating organic dyes into the silica

shells using a modified sol-gel procedure.18 In this case,

the dyes had a thioisocyanate functional group that could

be coupled to the amine group of 3-aminopropyl-triethoxy-

silane (APS, Aldrich) through an addition reaction. The

covalent bond formed in this reaction could stabilize the

fluorescent dye and make it possible to chemically incor-

porate this dye into the silica shell by cohydrolyzing with

TEOS. Two fluorescent dyes were selected as examples to

demonstrate the concept: 7-(dimethylamino)-4-methylcou-

marin-3-isothiocyanate (DACITC) and tetramethylrhodamine-

5-isothiocyanate (5-TRITC) (Molecular Probes, Eugene,

OR). In a typical procedure, 0.2 10-3 g of DACITC (or

5-TRITC) was added to a mixture of 0.5 mL APS coupling

agent and 5 mL 2-propanol after this mixture had been

degassed for 10 min. The reaction was allowed to proceed

at room temperature for 24 h in the dark under the protection

of nitrogen gas. The as-synthesized APS-DACITC compound

was mixed with TEOS precursor (1:4, v/v) and then injected

into the ferrofluid solution to form core-shell nanoparticles.

DACITC has its excitation and emission maxima at 400

and 476 nm. 5-TRITC has its excitation and emission

maxima at 543 and 571 nm. Figure 3A and B shows the

fluorescence optical microscopy images of DACITC- and5-TRITC-labeled core-shell samples taken with a Leica

inverted optical microscope (DMIRBE). These two samples

were prepared by evaporating 10 L of as-synthesized

nanoparticle dispersions on silicon substrates in the presence

of a 27 megagauss magnetic field (Polysciences, Warrington,

PA). These magnetic nanoparticles had been lined up to form

chain-like structures (with their longitudinal directions

oriented along the magnetic field) due to the attractive

interaction between the magnetic moments. The insets are

TEM images of the corresponding nanoparticles (deposited

on TEM grids under no magnetic filed), showing the core-

shell structure for these two samples.

In summary, we have demonstrated a convenient method

for coating superparamagnetic nanoparticles of iron oxide

with uniform shells of amorphous silica. The thickness of

this silica coating could be easily controlled in the range of

2-100 nm by changing the concentration of the TEOS

precursor. In addition to the iron oxide nanoparticles

contained in commercial ferrofluids, this procedure has also

been successfully extended to magnetite nanoparticles syn-

thesized using a wet chemical method.19 In this case,

superparamagnetic core-shell nanoparticles with a similar

control over the structure and uniformity were obtained.20

The silica shells could also be labeled with fluorescent

organic dyes to generate multifunctional nanoparticles that

exhibit a unique combination of magnetic and optical

properties. We believe that the sol-gel process described

Figure 3. Fluorescent microscopy images of chain-like structuresformed by silica-coated iron oxide nanoparticles in the presenceof an external magnetic field. The silica coatings of these nano-particles had been derivatized with fluorescent organic dyes bycoupling (A) DACITC, and (B) 5-TRITC with the APS precursor.The insets show TEM images of these core-shell nanoparticlesthat were deposited on TEM grids under no magnetic field.

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here can also be extended to other metal oxide systems to

fabricate core-shell nanoparticles with various properties

for different applications.21 In addition to their uses as

dispersions in liquid media, those magnetic particles with

relatively thick shells could also serve as building blocks to

construct photonic crystals whose band gap properties could

be manipulated using an external magnetic field.21

Acknowledgment. This work has been supported in part

by a DARPA-DURINT subcontract from Harvard University,a Fellowship from the David and Lucile Packard Foundation,

and a Career Award from the National Science Foundation

(DMR-9983893). Y.X. is a Research Fellow of the Alfred

P. Sloan Foundation (2000-2002). Y.Y. and B.T.M. thank

the Center for Nanotechnology at the UW for a Graduate

Student Fellowship and an IGERT Fellowship (supported

by the NSF, DGE-9987620), respectively.

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